1. Field of the Invention
The present invention relates to a joint structure for pillars such as, for example, foundation piles driven into the ground.
2. Related Background Art
Foundation piles are driven while being sequentially joined to each other to be thus penetrated into the ground. The conventional joint structure for such foundation piles has been of a welded joint type. The welded joint, however, is disadvantageous for several reasons: it requires a qualified welding operator; the welding operation is affected by weather conditions; the welding strength is dependent on the skill of an operator; and the long welding time iS required in foundation piles driving, which prolongs the total working time.
To solve the above-described disadvantages, the same inventors disclosed previously a non-welded joint structure for foundation piles that are to be driven into the ground in Japanese Utility Model No. HEI 3-83224. In this joint structure, three main components are provided: circumferential grooves or projections on the pillars; a circumferentially divided cylindrical ring (hereinafter referred to "an inside ring") covers the pillars by engaging the grooves or projections; and an outer cylindrical ring (hereinafter referred to "an outside ring") set on the inside ring. The circumferential grooves (or circumferential projections) are provided around the outer peripheries of the pillars to be joined to each other in the vicinity of the end portions thereof. A multi-divided cylindrical inside ring, which has an inside diameter portion including circumferential projections (or circumferential grooves) to be engaged with the above circumferential grooves (or circumferential projections) and having an outside diameter portion including a taper, is externally fitted around the joint portion of the above pillars. An integral outside ring having an inner surface with the same taper as that of the inside ring is fitted around the outside diameter portion of the inside ring. The outside ring is pushed in the longitudinal direction of the pillars to fasten the inside ring by the effect of the above' tapers of the inside ring and the outside ring, thereby joining the pillars to each other.
The above-described disadvantages of the welded joint have been solved by the development of a new joint, which utilizes in place of the welded joint the combination of the inside ring and the outside ring, that is, the so-called non-welded joint structure.
The above-described joint structure can be used with pillars having outside diameters ranging from 300 to 1200 mm. It has advantages in simplifying the assembly because no site welding is required, and standardized dimensions and accuracies of the joint permit easy manufacturing the joint with constant quality. The joint structure eliminates the difficulty in the works as in the welded joint, and enhances the reliability of the joint; accordingly, it has been favorably used.
In the above-described technique, however, it is necessary to enhance the accuracy in fitting between a plurality of the circumferential grooves (or circumferential projections) provided around the outside diameter portions of the pillars and a plurality of the circumferential projections (or circumferential grooves) provided around the inside diameter portion of the inside ring.
Incidentally, examination of past earthquake damage indicates that the liquefaction of the ground is generated in the stratum of a sand layer. When the ground is liquefied, the ground is often moved laterally at the position where the liquefaction is generated. The lateral movement of the ground occasionally extends several meters. When the ground is greatly moved laterally, the underground structure may float or sink, also it may be damaged or destroyed. The foundation piles, of course, may be broken in the ground.
The above-described non-welded joint structure is of a rigid joint type, that is, not of a flexible joint type, so that it cannot flexibly absorb the sideward movement of the ground due to an earthquake. Accordingly, pillars having the non-welded joint structure such as foundation piles buried in the ground may be possibly destroyed by earthquakes. Slender pillars such as foundation piles have a small bending toughness. The sideward movement of the ground is extremely dangerous for this structure. It is reported that pillars such as foundation piles buried in the ground are destroyed by the movement of the ground in an amount of several decimeters. This agrees well with the actual damage. Accordingly, there are has been strong demands for a new type joint structure for pillars, which is different from the conventional rigid joint type, and which is capable of being greatly deformed to follow the deformation of the ground.